1. Field of the Invention
The present invention relates to a toroidal-type continuously variable transmission which is improved in a retainer for rollably holding a plurality of rolling elements.
2. Description of the Related Art
In recent years, as a transmission for a car, or as a transmission for various industrial machines, there has been used a toroidal-type continuously variable transmission. And, as the toroidal-type continuously variable transmission, for example, there are known a transmission disclosed in Japanese Utility Model Unexamined Publication No. 6-16753 of Heisei in which a retainer of a power roller bearing is integrally formed of synthetic resin, a transmission disclosed in Japanese Utility Model Unexamined Publication No. 7-35847 of Heisei in which there is formed an oil groove in a retainer to thereby enhance the lubrication efficiency thereof, and a transmission disclosed in Japanese Patent Unexamined Publication No. 7-174146 in which there is formed an oil hole in a retainer to thereby enhance the lubrication efficiency thereof.
The retainer of the above-mentioned power roller bearing is generally structured in such a manner as shown in FIG. 8.
That is, on the periphery of a rotary shaft 1, there are rotatably supported an input disk 2 and an output disk 3 having their respective inner peripheral surfaces opposed to each other. Between the input and output disks 2 and 3, there is interposed a trunnion 4 which is capable of swinging about its pivot shaft (not shown) situated at a torsional position with respect to the center axes of the input and output disks 2 and 3 (Here, xe2x80x9ctorsional positionxe2x80x9d means a physical relation which lies in a direction at right angles to the direction of the rotary shaft 1 but does not intersect the rotary shaft 1). The trunnion 4 includes a displacement shaft 5 and, on the periphery of the displacement shaft 5, there is disposed a power roller 6 which is rotatably supported in such a manner that it is held between the input and output disks 2 and 3. Further, between the power roller 6 and trunnion 4, there is interposed a thrust rolling bearing 7 which is used to receive a thrust-direction load applied to the power roller 6.
The respective inner peripheral surfaces 2a and 3a of the input and output disks 2 and 3 are formed as concave surfaces each having an arc-shaped section, while the peripheral surface 6a of the power roller 6 is formed as a spherically convex surface; and, the peripheral surface 6a is in contact with the inner peripheral surfaces 2a and 3a. The thrust rolling bearing 7 comprises a plurality of rolling elements 8 and a retainer 9 for rollably holding the plurality of rolling elements 8.
The retainer 9 is composed of a circular-ring-shaped main body 10 and a plurality of pockets 11 which are respectively disposed in the intermediate portions of the main body 10 in the diameter direction thereof to rollably hold the rolling elements 8. Further, the retainer 9 includes a plurality of lubricating oil passages 12 respectively formed as recessed grooves which are interposed between the inner and outer peripheries of the main body 10 in such a manner that they cross the pockets 11.
Therefore, according to the above-structured toroidal-type continuously variable transmission, even when the retainer 9 forming the thrust rolling bearing 7 shifts in the axial direction to thereby cause one surface of the retainer 9 to come into close contact with a surface opposed to the present surface of the retainer 9, a sufficient amount of lubricating oil is allowed to flow through the lubricating oil passage 12 into the pockets 11 holding the rolling elements 8. As a result of this, there is eliminated a fear that a part of the thrust rolling bearing 7 can wear excessively or can be seized to its adjoining member.
However, in the thrust rolling bearing 7 used as the power roller bearing of the toroidal-type continuously variable transmission, due to its structure designed for traction contact, as shown in FIG. 8, between the thrust rolling bearing 7 and input and output disks 2 and 3, there can be obtained only two contact points (loading points) respectively shown by arrow marks in FIG. 9, while the two contact points have a contact angle of xcex1 between them. Therefore, the inner race 7a of the thrust rolling bearing 7 not only receives a force in the thrust direction but also generates a component of force in the radial direction at the 180xc2x0 opposite portion on the circumference thereof, so that the circular-ring-shaped thrust rolling bearing 7 is compressed in the radial direction.
Due to this compression, the inner race 7a is deformed into such an elliptical shape as shown in FIG. 9. Also, due to power transmission, in the traction contact portion, there is produced such a force 2Ft in the tangential direction as shown in FIG. 10A. This force turns into a force P which tends to fall down the thrust rolling bearing 7, as shown in FIG. 11B, thereby causing an imbalance in force.
In case where the thrust rolling bearing 7 is used in this condition, the rotational speeds of the rolling elements 8 around the retainer 9 show such distributions as shown in FIG. 11. That is, the rotational speeds of the rolling elements 8 (the lengths of arrow marks show the rotational speeds of the rolling element 8 around the retainer 9) in the 2Ft direction are lower than the rotational speeds of the rolling elements 8 in the anti-2Ft direction. Therefore, as shown in FIG. 12, the contact loads between the rolling elements 8 and retainer 9, in the anti-2FT direction, as shown by arrow marks (the lengths of the arrow marks represent the intensities of the contact loads), act so as to push the retainer 9 in its rotating direction but, in the 2Ft direction, acts so as to push the retainer 9 in the opposite direction to the rotating direction thereof. Accordingly, a compressive stress is applied to a pocket 11a, whereas a tensile stress is applied to a pocket 11b; and thus, during one rotation of the retainer 9, one pocket 11 receives one cycle of two-way stress loads ranging from the compressive stress to the tensile stress.
Also, conventionally, the lubricating oil passage 12 of the retainer 9 which is used under these conditions, as shown in FIGS. 13A and 13B, is formed of a recessed groove (a substantially U-shaped groove which includes two corner portions each formed as an arc R). Therefore, in case where a tensile stress is applied to the pocket 1, around the recessed groove, there is obtained such stress distribution as shown in FIG. 14; that is, there is a fear that the maximum stress X can be applied to the vicinity of the connecting portions between the two corner portions R and the bottom of the groove, thereby causing the retainer 9 to break around and from such connecting portions.
Also, the thrust rolling bearing 7 used as a power roller bearing in a toroidal-type continuously variable transmission handles the rolling elements 8 and retainer 8 as sub-assembling members in an intermediate step, in order to facilitate check and delivery to thereby reduce the manufacturing cost of the bearing in its assembling step. For this purpose, there is employed a so called xe2x80x9cball guide systemxe2x80x9d in which the retainer 9 is positioned by the rolling elements 8. In this system, no slide guide surface is provided for the inner and outer races to thereby be able to lower the dynamic torque loss of the bearing. This is especially important in the toroidal-type continuously variable transmission which is required to provide a high power transmission efficiency. In the ball guide system, a pocket clearance is important. In an ordinary bearing as well, the pocket clearance must be set not too large nor too small. Especially, in a power roller bearing for use in a toroidal-type continuously variable transmission, to the pockets of the retainer, there is applied a different force from that in an ordinary thrust ball bearing and, therefore, the pocket clearance of the power roller bearing must be set differently from the ordinary bearing.
In case where the clearance is set excessively small, a force applied to the retainer 9 tends to increase due to the difference between the rotational speeds of the rolling elements 8 around the retainer 9, thereby increasing the amplitude of the stress, with the result that the retainer 9 can be damaged due to the repeated stress fatigue. On the other hand, in case where the clearance is set excessively large, the retainer 9 can shake while rotating, thereby increasing the collision force between the retainer 9 and rolling elements 8, so that the retainer 9 can be damaged and the rolling elements 8 can peel off.
In the thrust rolling bearing 7 used as a power roller bearing, differently from the ordinary thrust bearing, the inner race 7a of the thrust rolling bearing 7 transmits power and, therefore, there is exerted a traction force which is a radial force. This radial force is supported by a needle roller bearing interposed between the inner race 7a and displacement shaft 5. However, the needle roller bearing requires a proper clearance and, thus, the inner and outer races tend to shift from each other by an amount equivalent to this clearance. Due to this, the rolling elements 8 vary only slightly in the contact angle depending on the positions of the rolling elements 8.
The rotational speed of each rolling element 8 of the power roller bearing around the retainer can be expressed by the following equation: that is,
xcfx89c=(1xe2x88x92Da/dm cosxcex1) ni/2
where Da expresses a ball diameter, dm expresses the pitch diameter of the rolling element, and ni expresses an inner race rotational speed, respectively. The rotational speed of the rolling element 8 around the retainer varies according to the contact angles xcex1, that is, in the power roller bearing, the rotational speed varies according to the positions of the rolling element 8. Due to such difference between the rotational speeds of the rolling element 8, the rolling element 8 can be butted against and moved away from the retainer 9, thereby applying a force to the retainer 9.
As described above, the position of the rolling element 8 is caused to shift in the circumferential direction thereof according to the positions (orientation angles) of the rolling element 8 in the circumferential direction thereof. Here, a graphical representation in FIG. 7 shows the amounts of such shift in the position of the rolling element 8. In case where the shift amount is larger than the clearance between the pocket 11 and rolling element 8, the rolling element 8 presses against the retainer 9, so that such pressing force is repeatedly applied to the retainer 9 to thereby cause the retainer 9 to be damaged.
The parts structure of the above power roller bearing, except for the power roller 6 provided in the inner race 7a, is almost similar in appearance to the thrust ball bearing used to support the rotary shaft 1 onto which the thrust load is applied. Accordingly, it has been studied whether the power roller bearing of a toroidal-type continuously variable transmission can be produced at a low cost by diverting parts designed for use in an existing thrust ball bearing.
However, it is true that the parts structure of the power roller bearing is quite similar in appearance to the thrust ball bearing, but the power roller bearing is quite different in the function of the inner race 7a from the ordinary thrust ball bearing. Due to this, in the power roller bearing, the distribution of loads applied to the inner race 7a itself as well as the contact behaviors between the rolling elements 8, which are interposed between the inner and outer races, and the inner and outer races are greatly different from those of the ordinary thrust ball bearing. Therefore, it is indispensably necessary to make various improvements in the power roller bearing with such differences from the ordinary thrust ball bearing into account.
For example, in the ordinary thrust ball bearing, the inner race is used as a support member for supporting the shaft of the thrust ball bearing. On the other hand, in the power roller bearing, the power roller 6 rotating integrally with the inner race 7a is a power transmission member which is used to transmit the rotation of the input disk 2 to the output disk 3 and corresponds to a speed change gear of a gear-type multistage transmission. And, since the power roller 6 is rotated at a high speed in such a state that it receives a strong pressure from the input and output disks 2 and 3, it generates great heat and such great heat generated in the power roller 6 heats the inner race 7a and rolling elements 8. For this reason, as lubricating oil to be supplied into between the inner and outer races, it is necessary to use high-viscosity traction oil which has been developed specially for the purpose of power transmission.
Also, the traction portions, where the power roller 6 comes into contact with the input and output disks 2 and 3, provide mutually opposing positions which are spaced 180xc2x0 apart from each other on the outer peripheral edge of the power roller 6, so that the strong pressures from the input and output disks 2 and 3 are concentratedly applied onto these mutually opposing positions (traction portions) as radial loads. Therefore, in the traction portions where the power roller 6 is contacted with the input and output disks 2 and 3, there is generated a very high contact surface pressure.
For example, the ordinary bearing is used under the contact surface pressure of 2 to 3 GPa or less; and, on the other hand, in the power roller bearing used in a toroidal-type continuously variable transmission, in normal deceleration, the contact surface pressure thereof increases up to a pressure in the range of 2.5 to 3.5 Gpa and, in the maximum deceleration, there is a possibility that the contact surface pressure can reach a pressure as large as 4 GPa.
Further, the strong pressures given from the input and output disks 2 and 3 are concentratedly applied as the radial loads at the 180xc2x0 spaced mutually opposing positions on the traction portions of the power roller 6, so that the power roller 6 and the inner race 7a with the power roller 6 mounted thereon are compressed and deformed in the radial direction thereof. Since such compressive deformation warps the inner race 7a, it is almost impossible that the thrust loads applied onto the inner race 7a from the power roller 6 can be shared uniformly among the plurality of rolling elements 8 respectively interposed between the inner race 7a and outer race. That is, the thrust load acts concentratedly on some of the rolling elements 8 which are situated in the mutually opposing positions, with the result that the contact surface pressure of the rolling elements 8 with the raceway grooves is caused to vary and thus such some of the rolling elements 8 are caused to roll on the raceway grooves with a very high contact pressure.
Therefore, it is indispensable that the traction portions to come into contact with the input and output disks 2 and 3 as well as the raceway grooves of the inner and outer races to be contacted by the rolling elements 8 must be specially adjusted in the material, surface hardness, and surface roughness thereof in order to prevent the reduced life of the power roller bearing caused by the localized action of the high-contact surface pressure.
From the above background, the present applicants have already proposed a technology in which, in order to enhance the durability of the power roller bearing against the localized action of the high contact surface pressure to thereby be able to enhance the life of the power roller bearing, the rolling elements 8 are respectively made of middle carbon steel or high carbon steel as well as the surface hardness and strength of the rolling elements 8 are adjusted using carbonitrising treatment and hardening and tempering treatment (see Japanese Patent Unexamined Publication No. 7-208568 of Heisei).
Also, the present applicants have proposed a technology in which the input and output disks 2 and 3 as well as the power roller 6 and inner race 7a to be contacted with the input and output disks 2 and 3 are carburized and, after then, are ground or finished, or they are carbonitrided and, after then, are ground or finished to thereby adjust the hardness and effective hardened layer depth of the surfaces of these members to a proper value (in the range of 2 to 4 mm) capable of standing against the action of the localized contact surface pressure (see Japanese Patent Unexamined Publication No. 7-71555 of Heisei).
However, although the traction oil is employed as the lubricating oil to be supplied into between the inner and outer races, and the power roller 6, inner race 7a and rolling elements 8 are made of selected materials and surface treated to thereby adjust the surface hardness, effective hardened layer depth and surface roughness to their respective proper conditions, the desired lives of the above-mentioned traction portions and inner and outer races raceways of the power roller bearing cannot be obtained in a sufficient level.
Since the original object of the power roller bearing is to transmit power, it is important that the dynamic torque loss within the bearing can be reduced as much as possible to thereby enhance the torque transmission efficiency. However, only by the above-mentioned improvements, there is still left a possibility that, depending on the setting of the dimensions of the raceway grooves on the inner and outer races as well as the rolling elements 8, the dynamic torque loss within the bearing can increase to thereby lower the torque transmission efficiency.
Also, in spite of the above-mentioned proper adjustment of the hardness and effective hardened layer depth of the surfaces of the power roller 6 and inner race 7a, there is still left a possibility that, the edges of the raceway grooves and rolling elements 8 can be damaged early or the contact surfaces of the raceway grooves and rolling elements 8 can be damaged to thereby lower the life of the bearing.
The present invention aims at eliminating the drawbacks found in the above-recited conventional toroidal-type continuously variable transmissions. Accordingly, it is an object of the invention to provide a toroidal-type continuously variable transmission which can relieve the distribution of stresses applied to a plurality of lubricating oil passages formed in a retainer used therein, and further can secure the required strength of a retainer, can guide rolling elements properly, and thus can enhance the durability of the transmission.
According to one aspect of the invention, there is provided a toroidal-type continuously variable transmission, including: a rotary shaft; first and second disks rotatably supported on the periphery of the rotary shaft respectively, each of the inner surfaces of the first and second disks defining a concave surface having an arc-shape in section, the inner surfaces of the first and second disks being disposed opposed to each other; a trunnion swinging about a pivot shaft situated at a torsional position with respect to the center axes of the first and second disks; a displacement shaft provided in the trunnion; a power roller held between the first and second disks in such a manner as to be rotatably supported on the periphery of the displacement shaft, the power roller including a peripheral surface formed in a spherical convex surface; and a thrust rolling bearing interposed between the power roller and the trunnion for supporting a thrust-direction load applied to the power roller. The thrust rolling bearing includes a plurality of rolling elements and a retainer rollably holding the plurality of rolling elements. The retainer defines a circular-ring-shaped main body and a plurality of pockets respectively formed in the main body for rollably holding the plurality of rolling elements. A clearance between the pocket and the rolling element is set in the range of 0.6 to 6.0% of the ball diameter of the rolling element.
To secure the strength of the retainer, the retainer is designed in such a manner that its outside diameter is as large as possible unless the retainer interferes with the traction surface. Shaking of the retainer gives rise to the mutual contact between the outside diameter of the retainer and traction surface, thereby causing the traction surface, which is transmitting high power, to be damaged. According to a test using rolling elements each of about 16 mm, in case of a clearance of 0.1 mm or less, the retainer was damaged and, in case of a clearance of 0.9 mm or more, the rolling elements were heavily damaged. Therefore, the clearance between the pocket and rolling element may be preferably set in the range of 0.6 to 6.0% of the ball diameter of the rolling element.
Further, according to another aspect of the invention, there is provided a toroidal-type continuously variable transmission, including: a rotary shaft; first and second disks rotatably supported on the periphery of the rotary shaft respectively, each of the inner surfaces of the first and second disks defining a concave surface having an arc-shape in section, the inner surfaces of the first and second disks being disposed opposed to each other; a trunnion swinging about a pivot shaft situated at a torsional position with respect to the center axes of the first and second disks; a displacement shaft provided in the trunnion; a power roller held between the first and second disks in such a manner as to be rotatably supported on the periphery of the displacement shaft, the power roller including a peripheral surface formed in a spherical convex surface; and a thrust rolling bearing interposed between the power roller and the trunnion for supporting a thrust-direction load applied to the power roller. The thrust rolling bearing includes a plurality of rolling elements and a retainer rollably holding the plurality of rolling elements. The retainer defines a circular-ring-shaped main body, a plurality of pockets respectively formed in the main body for rollably holding the plurality of rolling elements and a plurality of lubricating oil passages respectively formed between the inner and outer peripheral edges of the main body in such a manner as to cross the pockets respectively. Each of said lubricating oil passages having a section formed in a single arc shape.
According to the above structure, the forcibly supplied lubricating oil passes through the lubricating oil passages formed in the retainer to thereby be able to lubricate the thrust rolling bearing. Also, since the section of each of the lubricating oil passages has a single arc shape, there can be relieved stresses which are generated in the lubricating oil passages.